The present application relates to double-base bipolar transistors and circuits and systems including them, and to methods for operating such transistors and circuits and systems.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Application Ser. No. 14/313,960 (now issued as U.S. Pat. No. 9,029,909, and hereby incorporated by reference) disclosed bidirectional bipolar transistors known as “B-TRANs,” and methods for operating such transistors. Further improvements, applications, and implementations were disclosed in subsequent commonly-owned and co-pending patent applications, including e.g. US/2016-0141375, US/2016-0173083, WO/2016-112395, and US/2016-0241232, all of which are hereby incorporated by reference.
The most basic B-TRAN is a four-terminal three-layer power device. In the most basic version, each side of a p-type monolithic semiconductor wafer has an n-type emitter/collector region and a p-type base contact region. The two base regions are contacted separately, and the two emitter/collector regions are connected to provide the two current-carrying terminals. This example can operate, in either direction of current flow, as an NPN bipolar transistor.
The polarity of the externally applied voltage will determine which of the emitter/collector regions is operating as the emitter (i.e. is emitting electrons, in this NPN example), and which is operating as the collector. The two base regions are not connected together, but are operated separately: in describing the operation of this class of devices, the base region on the same surface as the collector will normally be referred to as the “c-base,” and the base region on the same surface as the emitter will normally be referred to as the “e-base.” (Of course, when the external polarity reverses, the functions of the two emitter/collector regions will be exchanged: the collector will become the emitter, the emitter will become the collector, the e-base will become the c-base, and the c-base will become the e-base.)
The methods of operation described in the U.S. Pat. No. 9,029,909 patent, and in the published applications listed above, result in a fully bidirectional switching device which provides high gain, high breakdown voltage, and low voltage drop in the ON state. This combination of advantages is extremely advantageous.
To achieve this combination of advantages, an operation sequence was disclosed which included a pre-turnoff timing phase as well as a preliminary turn-on phase (known as “diode turn-on”). The principles of operation are very different from those of conventional bipolar power transistors, as may be seen from the following description.
A B-TRAN is in the “active off-state” when the e-base (base on the emitter side) is shorted to the emitter, and the c-base (base on the collector side) is open. In this state, with the NPN B-TRAN, the collector is the anode (high voltage side), and the emitter is the cathode (low voltage side).
The B-TRAN is also off when both bases are open, but due to the high gain of the B-TRAN in this state, the breakdown voltage is low. The series combination of a normally-ON JFET and a Schottky diode attached between each base on its respective emitter/collector, as previously disclosed, will significantly increase the blocking voltage in this “passive off-state”. The JFETs are turned off during normal operation.
The e-base is essentially at a constant voltage—it varies only about 0.1 V from a low drive to a high drive condition. The c-base, in contrast, is a nearly constant current drive, even as the voltage is varied from 0 V above the collector to about 0.6 V. Instead of the c-base current changing with c-base voltage, Vce changes. At a c-base voltage of 0 V (c-base shorted to collector), there is a certain gain that depends on the emitter current density, and Vce is nominally 0.9 V over a large range of current density. Raising the c-base to 0.1 V above the collector does not change the gain, but it lowers Vce by nominally 0.1 V. Raising the c-base to 0.6V drops Vce to about 0.2 or 0.3 V.
One sample embodiment for B-TRAN turn-on is to simultaneously, from the active off-state and blocking forward voltage, open the e-base-to-emitter short while shorting the c-base to the collector. This immediately introduces charge carriers into the highest field region of the depletion zone around the collector/base junction, so as to achieve very fast, forward biased turn-on for hard switching, very similar to IGBT turn-on.
Another advantageous turn-on method, from the active off-state, is to have the circuit containing the B-TRAN reverse the B-TRAN polarity, which produces the same base state described in the hard turn-on method, but at near zero voltage. That is, the e-base which is shorted to the emitter becomes the c-base shorted to the collector as the B-TRAN voltage reverses from the active off-state polarity. Again, turn-on is fast.
In a third turn-on method from the active off-state, the e-base is disconnected from the emitter, and connected to a current or voltage source of sufficient voltage to inject charge carriers into the base region. This method is likely slower, since the charge carriers go into the base just below the depletion zone. Also, it is known that carrier injection into the e-base results in inferior gain relative to carrier injection into the c-base.
After turn-on is achieved with either of the methods using the c-base, Vce is more than a diode drop. To drive Vce below a diode drop, turn-on goes to the second stage of increased charge injection into the c-base via a voltage or current source. The amount of increased charge injection determines how much Vce is reduced below a diode drop. Injection into the e-base will also reduce Vce, but the gain is much lower than with c-base injection.
In the first step of one advantageous turn-off method, the c-base is disconnected from the carrier injection power supply and shorted to the collector, while the previously open e-base is shorted to the emitter. This results in a large current flow between each base and its emitter/collector, which rapidly removes charge carriers from the drift region. This in turn results in a rising Vce as the effective resistivity of the drift region increases. At some optimum time after the bases are shorted, the connection between the c-base and the collector is opened, after which Vce increases rapidly as the depletion region forms around the collector/base junction.
Turn-off can be achieved by simply opening the c-base and shorting the e-base to the emitter, but this will result in higher turn-off losses since the drift region (base) will have a high level of charge carriers at the start of depletion zone formation.
Turn-off can also be achieved by simply opening the c-base and leaving the e-base open, but this will result in the highest turn-off losses and also a low breakdown voltage.
The present application teaches, among other innovations, new improvements to methods of operating a B-TRAN-type device, and new circuits which perform these improved methods. The present application also teaches, among other innovations, circuits and systems which incorporate a device with improved operation as above, and methods for operating such circuits and systems. A particularly beneficial feature is the introduction of an additional pre-turnoff timing phase, as described below. This additional timing phase reduces the minority carrier population, resulting in faster quenching of bipolar conduction.
In one example of an NPN B-TRAN device, turn-off begins with a pre-turnoff stage as before, where each base contact region is shorted to its adjacent emitter/collector region. However, according to the additional disclosure in the present application, this first pre-turnoff stage is followed by a second pre-turnoff timing phase, where negative drive is applied to the e-base (i.e. to the base contact region on the same side as the emitter, which is the more negative of the two emitter/collector regions). This negative drive reduces the population of holes in the bulk base (which is the p-type bulk of the semiconductor material). Since the population of holes is reduced, secondary emission of electrons from the collector junction is also necessarily reduced, and the nonequilibrium ON-state carrier concentration moves toward its equilibrium value. (The nonequilibrium carrier concentration can be orders of magnitude greater than its equilibrium value.)
Some of the disclosed base drive circuits provide this second pre-turnoff timing phase very easily, since the base drive circuits disclosed in previous applications already included elements which provided the voltage offset which is exploited here to provide negative base drive (for an NPN device; of course, polarities are reversed in a PNP device).
The innovative teachings provide the benefit, among others, of faster turn-off, and correspondingly less energy loss, in double-base bipolar transistors.
The innovative teachings also provide more efficient switching of phase legs (and analogous configurations). When two transistors are connected in series between two supply lines, the transistor which is turning off (during switching) will turn off faster during its reverse recovery: this reduces the current which would pass through the other transistor of the phase leg while it is turning on.
Further inventive features and advantages are set forth in the following description.